A conveyor comprises a track, along which a load is moved with the conveyor. The track of a conveyor has e.g. pallets, steps or corresponding to be moved, supported on which the load to be transferred, such as passengers or freight, is moved. Moving of the load occurs with drive machinery, which in addition to an electric motor can comprise e.g. a rotating gear wheel, which connects mechanically to the track of the conveyor, thus transmitting the force needed for moving the load. The power supply to the electric motor of the drive machinery normally occurs from an electricity network with a power supply apparatus, such as a frequency converter. The drive machinery with its power supply apparatus can be disposed e.g. in the end zone of the conveyor at the change-of-direction point of the track of the conveyor.
The power supply apparatus of the conveyor can be e.g. a frequency converter that returns electric power to the network. By means of the frequency converter the power to be supplied from the electricity network to the drive machinery of the conveyor, and thus also the running speed of the conveyor, can be adjusted steplessly. Since the power requirement of the conveyor is also proportional to the running speed of the conveyor, as well as to the load to be moved, by reducing the running speed of an idly running conveyor the energy consumption of the conveyor system can be minimized without this having a substantial effect on the transfer capacity of the conveyor. In this case when it is detected that the load of the conveyor is increasing, the running speed of the conveyor can be raised back to the normal operating speed of the conveyor. A problem with this solution is that the power-handling capacity of the frequency converter must be dimensioned according to the full load of the conveyor and according to the power requirement of the normal operating speed. In this case the size of the frequency converter as well as the size of the cooling apparatus needed increases to be quite large, in which case in addition to power losses and possible heat transfer problems, one problem is also the space in the conveyance system that needs to be reserved for the frequency converter/cooling apparatus.
Owing to the aforementioned reasons, inter alia, solutions have been developed wherein the electricity supply to the drive machinery of the conveyor occurs during heavy loading of the conveyor directly from the electricity network by connecting the phases of the network to the supply leads of the drive machinery with contactors. After the load of the conveyor has decreased, the contactors are opened and the power supply to the drive machinery of the conveyor is continued with the frequency converter, in which case the running speed of the conveyor and therefore the energy consumption of the conveyor can still be reduced with the frequency converter; the frequency converter to be used can in this case, however, be dimensioned for a significantly smaller power requirement, so that the size of the frequency converter/cooling apparatus decreases. One such solution is presented in publication U.S. Pat. No. 4,748,394.
Before the frequency-controlled drive machinery of a conveyor that operates at reduced speed can be connected to the electricity network with contactors, the running speed of the conveyor must first be increased to the normal operating speed and such that the frequency and phase of the current to be supplied to the drive machinery with the frequency converter can be synchronized with the network. Users of the conveyor will detect even a small synchronization error as a nudge or as a downright impact, which is transmitted from the drive machinery to the track of the conveyor in connection with the closing of the contactors.
A similar nudge is also detected if the amplitude of the voltage supplied by the frequency converter was smaller than the amplitude of the network voltage a moment before the closing of the contactors. In order to avoid this the amplitude of the supply voltage of the frequency converter can also be increased to near the amplitude of the network voltage for the purpose of synchronization with the network. Since synchronization with the network/increasing of the amplitude of the voltage supplied must, owing to the underdimensioned frequency converter, be performed before the loading of the conveyor has increased too much, from the moment of the closing of the contactors onwards the drive machinery of the conveyor operates at least for some time overexcited with an unnecessarily large voltage.
Overexcitation increases the current requirement of the drive machinery and at the same time also the energy consumption of the conveyor increases. On the other hand, the loading of the conveyor can vary to some extent also with heavy loading during normal operation of the conveyor, so that also during normal operation overexcitation increases the energy consumption of the conveyor.